Formation of formic acid with the help of indium-containing catalytic electrode

ABSTRACT

Electrochemical conversion of CO 2  to formic acid or a salt thereof, using an indium containing catalytic electrode, comprising (a) electrochemically converting CO 2  to formic acid or a salt thereof by applying a voltage to an electrochemical cell comprising the catalytic electrode as cathode and an anode, wherein the electrochemical cell is fed with an electrolyte comprising CO 2 ; and (b) regenerating the catalytic electrode by lowering the voltage and subsequently washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air without applying voltage; and (c) optionally repeating steps (a) and (b).

FIELD OF THE INVENTION

The present invention is in the field of electrochemistry, especially inthe electrochemical conversion of carbon dioxide.

BACKGROUND ART

The electrochemical conversion of carbon dioxide into economicallyvaluable materials such as fuels and industrial chemicals orintermediate products thereof is gaining interest in view of mitigatingthe emission of carbon dioxide into the atmosphere, which is responsiblefor damaging effects such as climate alterations, changes in pH ofseawater, melting of polar ice and sea level rise. The electrochemicalIndium-containing catalysts for the electrochemical reduction of CO₂ toformate are known in the art, e.g. from WO 2019/141827, WO 2014/032000and WO 2014/042781. Catalyst stability is one of the major bottlenecksin electroconversion of CO₂ to valuable compounds such as formate.Solving this problem is therefore extremely important in order to enablethe technological viability and scale-up for industrially relevantprocesses to be developed. The present invention provides in the needfor an easy regeneration of indium-containing catalysts for theelectrochemical reduction of CO₂ to formate, which increased faradaicyields and electrode lifetime. US2013/105304 relates to a process forthe electrochemical conversion of CO₂ to formic acid or a salt thereofusing a high surface area cathode which may include an indium coatingand having a void volume of between 30 and 98%. The performance of thissystem may decrease with regard to formate yield which may result fromcatalyst loss or over-coating of the catalyst with impurities such asother metals that may be plated onto the cathode. The surfaces of thecathode may be renewed by the periodic addition of indium salts or a mixof indium/tin salts in situ during operation of the electrolyzer. Otheror additional metal salts may be added in situ as well. During injectionof the metal salts, the electrolyzer may be temporarily operated at alower current density with or without carbon dioxide addition.US2015/218716 relates to reduction of carbon dioxide to products in amethod wherein an indium cathode is oxidized. US2019/085477 relates toelectrochemical conversion of carbon dioxide. It is taught that thecatalyst can be refreshed by stopping the electrolytic voltage andreduction reaction, discharging the cathode solution and the anodesolution and supplying a rinse solution while applying a refreshvoltage. The refresh voltage may be cyclically applied so that theoxidation treatment of the ions and the impurities and the reductiontreatment are alternately performed. Next, gas is supplied to dry thecathode and anode. When the rinse solution is supplied to the cathodesolution flow path, the saturation degree of water in the gas diffusionlayer increases and output reduction occurs due to the diffusibility ofgas. By supplying the gas, the saturation degree of water lowers so thatthe cell performance is recovered and the refresh effect is increased.The exemplified process uses a membrane electro catalyst assembly (MEA).

SUMMARY OF THE INVENTION

The inventors have surprisingly found that an indium-containingcatalytic electrode for the formation of formic acid could readily beregenerated by exposure to air. Even more surprisingly, the Faradayyields obtained at the electrode were increased and the lifetime of theelectrode could be increased by this regeneration. The regenerationaccording to the invention can be employed in a process for theelectrochemical conversion of CO₂ to formic acid or a salt thereof,comprising: (a) electrochemically converting CO₂ to formic acid or asalt thereof by applying a voltage to an electrochemical cell comprisingthe catalytic electrode as cathode and an anode, wherein theelectrochemical cell is fed with an electrolyte comprising CO₂; (b)regenerating the catalytic electrode by lowering the voltage andsubsequently washing the catalytic electrode with an aqueous liquid andexposing the catalytic electrode to air without applying voltage; and(c) optionally repeating steps (a) and (b). The regeneration accordingto the invention is able to provide an improved faradaic yield of theelectrochemical reaction and an improved lifetime of the electrode. Theinvention further concerns an electrochemical cell assembly for thecontinuous reduction of CO₂ to formic acid for regeneration according tothe invention. Voltage is not applied if there is a current of 0 mA/minin other words no current. Preferably, no voltage is applied during eachthe wash with aqueous liquid and exposure to air. It is preferred thatstep (b) comprises regenerating the catalytic electrode by lowering thevoltage followed by washing the catalytic electrode with an aqueousliquid and subsequently exposing the catalytic electrode to air withoutapplying voltage; and (c) optionally repeating steps (a) and (b). It ispreferred to repeat steps (a) and (b) in step (c).

An aspect of the invention can be defined as a process for theelectrochemical conversion of CO₂ to formic acid or a salt thereof,using an indium containing catalytic electrode, comprising: (a)electrochemically converting CO₂ to formic acid or a salt thereof byapplying a voltage to an electrochemical cell comprising the catalyticelectrode as cathode and an anode, wherein the electrochemical cell isfed with an electrolyte comprising CO₂; (b) regenerating the catalyticelectrode by lowering the voltage and subsequently washing the catalyticelectrode with an aqueous liquid and exposing the catalytic electrode toair without applying voltage; and (c) optionally repeating steps (a) and(b).

The catalytic electrode preferably is an indium-bismuth catalyst,indium-tin catalyst or an indium catalyst, more preferably anindium-bismuth catalyst. The process preferably is operated in cycleswherein step (c) is repeated at least 10 times, more preferably1000-1000000 times. In each cycle the duration of step (b) preferably is0.1-50%, more preferably 1-10%, most preferably 3-7% of the duration ofstep (a); and/or wherein the duration of step (a) in a single cycle isin the range of 1-100 h, preferably 20-30 h, and wherein the duration ofstep (b) in a single cycle is in the range of 0.1-2.5 h, preferably0.5-1.5 h, most preferably wherein the duration of step (a) in a singlecycle is 23 h and the duration of step (b) in a single cycle is in therange of 1 h. The exposure of the electrode to air in step (b)preferably is performed by feeding air to the electrochemical cell, morepreferably wherein the electrochemical cell is equipped with air jets.The catalytic electrode preferably is a gas diffusion electrode, morepreferably wherein air is led through the gas diffusion electrode duringstep (b). The process preferably is a continuous process, morepreferably a process wherein a plurality of electrochemical cells areconnected in parallel and wherein some of the cells are being subjectedto the regeneration of step (b) while other cells are simultaneouslyused for the conversion of step (a). The aqueous liquid for use in theprocess preferably is deionized water or the electrolyte used duringstep (a). The process preferably further comprises a control systemwhich determines the performance of the electrochemical cell, preferablyby determining the faradaic yield, and wherein step (a) is interruptedand step (b) is initiated in case the performance drops below apredetermined threshold value. Step (b) of the process preferablyinvolves (1) ramping down the current, preferably with a decrease of0.1-10 mA/min, more preferably 1-4 A/min, and no longer applying anexternal voltage; (2) stopping the liquid and gas flows; (3) washing thecatalytic electrode with the aqueous liquid; (4) feeding air, preferablyat a rate of 0.01-10 L/min, more preferably 0.05-0.5 L/min; (5) startingthe liquid and gas flows; (6) ramping up the current, preferably with anincrease of 0.1-10 mA/min, more preferably 1-4 A/min. A further aspectof the invention is use of a regeneration step for improving thefaradaic yield of an electrochemical process, wherein the regenerationinvolves lowering the voltage, washing the catalytic electrode with anaqueous liquid and exposing the catalytic electrode to air, and whereinthe electrochemical process comprises the conversion of CO₂ to formicacid or a salt thereof, using an indium-containing catalytic electrode.

Another aspect is use of a regeneration step for improving the lifetimeof an indium-containing catalytic electrode, wherein the regenerationinvolves lowering the voltage, washing the catalytic electrode with anaqueous liquid and exposing the catalytic electrode to air, and whereinthe electrode is used for an electrochemical process comprising theconversion of CO₂ to formic acid or a salt thereof.

A further aspect is an electrochemical cell assembly for the continuousreduction of CO₂ to formic acid, comprising a plurality ofelectrochemical cells, each cell comprising an anode and anindium-containing catalytic gas diffusion electrode as cathode, whereinthe cathode is configured to receive either an electrolyte containingCO₂ or air, and wherein each cell further contains an outlet fordischarging formic acid or a salt thereof wherein the gas diffusionelectrodes are equipped with an air jet stream to enable contacting ofthe electrode with air during regeneration. The electrochemical cellassembly preferably comprises a plurality of electrochemical cellsarranged in blocks each containing an equal amount of electrochemicalcells, preferably 1-25 electrochemical cells, most preferably 1 or 10electrochemical cells, wherein each block alternates between a firstposition wherein it is used for conversion of CO₂ to formic acid or asalt thereof and a second position wherein it is regenerated.Preferably, each electrochemical cell contains a cathode compartment andan anode compartment separated by at least one membrane and wherein thecathode compartment contains an inlet for receiving either theelectrolyte containing CO₂ or air and the anode compartment a separateinlet for receiving an anolyte.

DETAILED DESCRIPTION

Electrochemical cells are well-known in the art. They are equipped withan anode and a cathode and may comprise one or more semi-permeablemembranes located in between the anode and cathode, as such forming ananode compartment and a cathode compartment. In operation, an oxidationreaction occurs at the anode and a reduction reaction occurs at thecathode. Indium-containing catalysts for the electrochemical reductionof CO₂ to formate are known in the art. These catalytic electrodes areideally suitable as cathode.

The present inventors have developed a process for the regeneration ofsuch an indium-containing electrode. The regeneration process accordingto the invention not only regenerates the electrode, but even improvesits performance, in particular the faradaic yield, and its lifetime.Hence, in one aspect the regeneration according to the present inventionis used to improve the faradaic yield of the electrode. Alternatively,in one aspect the regeneration according to the present invention isused to improve the lifetime of the electrode. In another aspect, thepresent invention concerns a process for the electrochemical reductionof CO₂ to formate, making use of the regeneration according to thepresent invention. The invention further concerns an electrochemicalcell assembly for the continuous reduction of CO₂ to formic acid, whichis specifically designed to execute the regeneration according to thepresent invention. The definition of the electrode and the regenerationprocess according to the present invention are common to all aspects ofthe present invention.

Without being bound to a theory, the inventors believe that thecatalytic layer is freed from deposits that block the catalyst duringthe washing step. The washing liberates the metal layer, which isactivated by exposure to air. It is believed that the catalytic activesites of the catalyst may be rearranged during exposure to air. Since acolour change was observed after air regeneration, it is assumed thecatalyst is at least partially oxidized during regeneration, leading tosuch structural and/or morphological rearrangement. The increase infaradaic yield after a regeneration step may only occur after severalminutes or even an hour or so, indicating that the rearrangement may betriggered by air (oxidation), but occurs slowly.

The Electrode

The electrode in the context of the present invention is anindium-containing catalytic electrode for the electrochemical reductionof CO₂ to formate.

In addition to indium, the electrode may contain further elements, suchas one or more elements selected from the group consisting of C, Pt, Pd,Rh, Mo, Zr, Nb, Os, Au, Ag, Ti, Cu, Ir, Ru, Re, Hg, Pb, Ni, Co, Zn, Cd,Sn, Fe, Cr, Mn, Ga, TI, Sb, Ga and Bi, preferably from the groupconsisting of Sn, Pb, Ga and Bi. The atoms are typically present intheir metallic form, although oxides have also been known to reducecarbon dioxide. The cathode may contain further components, such asligands to stabilize the metal atoms and/or to catalyse the reduction ofCO₂, e.g. hydrides, halides, phosphines and porphyrins. Single metalIndium-cathodes may be used as well as alloys. Indium-containing alloyshave been found particularly effective in the reduction of 002.Preferred electrodes are selected from indium electrodes, indium-bismuthelectrodes, indium-tin electrodes and indium-lead electrodes. In apreferred embodiment, the catalytic electrode is an indium-bismuthcatalyst, indium-tin catalyst or an indium catalyst, most preferably anindium-bismuth catalyst.

In a preferred embodiment, the catalytic electrode is an indium-bismuthelectrode, wherein the amount of bismuth is in the range of 5-94 wt. %based on the total amount of bismuth and indium, preferably in the rangeof 10-90 wt. %, more preferably 30-90 wt. %, such as 35-90 wt. %, mostpreferably in the range of 40-60 wt. %, such as 45-55 wt. %. Such ratioshave shown to provide improved catalytic properties regarding carbondioxide to formate conversion, see e.g. WO 2019/141827. The catalyst cancomprise a combination of bismuth and indium in different thermodynamicphases.

The electrode may contain a porous support. The porous support allowsgas and liquid to interact. The porous support may be structured as afoam, felt and/or mesh. The electrode can consist of the catalyticmaterial, but the catalytic material may also be deposited on a support,such as a carbon support. Preferably, the catalyst is applied incombination with an electrically conductive support. As a conductivesupport a particulate material, in particular carbon particles, may beused. Preferably the conductive support comprises a porous structure ofcarbon particles bonded together. A preferred binding material is ahydrophobic binder, such as a fluorinated binder. The catalyst isdeposited onto or adhered to the conductive material. The weight ratioof indium and bismuth to carbon can advantageously be in the range of0.10-1.50, preferably 0.2-0.8.

In a preferred embodiment, the electrode is a gas diffusion electrode(GDE). Gas diffusion electrodes are highly suitable for the reduction ofCO₂, especially when CO₂ in gaseous form is used as electrolyte. Agas-diffusion electrode provides a high surface area or interface forsolid-liquid-gas contact. Such a gas-diffusion electrode typicallycomprises an electrically conductive substrate, which may serve as asupporting structure for a gas-diffusion layer. The gas-diffusion layerprovides a thin porous structure or network, e.g. made from carbon, forpassing a gas like carbon dioxide from one side to the other. Typicallythe structure is hydrophobic to distract water. The gas diffusion layermay comprise the catalytically active material. By diffusion of gaseousCO₂ through the pores of the cathode, the area that is available forreducing CO₂ is maximized, as such increasing the overall efficacy ofthe process according to the invention. Additionally, the same gas inletcan be used to receiving air during the regeneration according to thepresent invention.

The gas diffusion electrode typically contains the indium-containingcatalytic system embedded in the gas-diffusion layer or provided as oneor more additional separate layers thereof. Examples of suitablesubstrates include metal structures like expanded or woven metals, metalfoams, and carbon structures including wovens, cloth and paper. Asexplained above, the conductive support for the catalyst is preferablyformed by particulate carbon. The catalyst system is preferably bondedto the electrically conductive substrate using a hydrophobic binder,such as PTFE.

Regeneration

The regeneration according to the present invention involves loweringthe voltage. After lowering of the voltage, no external voltage is to beapplied during the remainder of the regeneration. Subsequently, thecatalytic electrode is washed with an aqueous liquid and exposed to air.The exposure of the electrode to air is typically performed by feedingair to the electrochemical cell, preferably wherein the electrochemicalcell is equipped with air jets which are configured to feed the air tothe electrochemical cell. Preferably, the feeding of air is directly tothe electrode. In case the electrode is a gas diffusion electrode,exposure to air is conveniently performed by leading air through the gasdiffusion electrode. In case the air originates from a compressor, it ispreferred that the air is first led through a filter to remove any oilor other particles, before using the air for regeneration.

Before exposure to air, the catalytic electrode is washed with anaqueous liquid. Without being bound to a theory, the washing step wasfound to be essential for liberating the metal atoms (indium andpossibly other metal(s)) before exposure to air. The water of theaqueous liquid may be pure (deionized) water or may contain othercomponents, such as inert gases (e.g. N₂, Ar), H₂, bases such asbicarbonate and/or formate (e.g. potassium or sodium salts). Preferably,the aqueous liquid should be essentially free from divalent cations suchas Ca²⁺ and Mg²⁺. In a preferred embodiment, the aqueous liquid isdeionized water or the electrolyte used during step (a). Mostpreferably, deionized water is used.

The duration of the exposure to air may be in the range of 1 min-24 h,preferably 0.1-10 h. optimal results have been obtained withregeneration for about 18 h but also with regeneration for about 1 h intotal (air flow for about 30 min), indicating that the exact duration ofthis step is not crucial. In one embodiment, the duration of step (4) is0.5-1.5 h, most preferably about 1 h. The air flow may have a flow rateof 0.1-100 mL/min per cm² electrode surface area, more preferably 0.5-10L/min per cm² electrode surface area, most preferably 1-3 L/min per cm²electrode surface area. Such flow rate and duration have been foundparticularly suitable to regenerate a gas diffusion electrode.

The regeneration according to the present invention preferably involvesin the indicated sequence:

-   (1) ramping down the current;-   (2) stopping the liquid and gas flows;-   (3) washing the electrode;-   (4) feeding air, preferably at a rate of 0.01-10 L/min, more    preferably 0.05-0.5 L/min;-   (5) starting the liquid and gas flows;-   (6) ramping up the current.

The regeneration process according to this embodiment preferably startswith lowering the current such that the electrochemical conversion ishalted. In step (1), the current is lowered, preferably with a decreaseof 0.1-10 mA/min, more preferably 1-4 A/min. At the end of step (1), thecurrent is typically reduced to 0 mA/min and no external voltage isapplied. If the electrode is part of a gas diffusion electrode, thewashing fluid of step (3) and air of step (4) preferably are fed to theoutward facing surface of the indium containing electrode. The outwardfacing surface is the surface which is in first contact with the cathodeelectrolyte.

An electrochemical process typically involves the feeding of one or moreelectrolytes to the electrochemical cell. An electrolyte may be gaseousand/or liquid. In the context of reducing CO₂, the electrolyte containsa source of CO₂, in which case the electrolytes are often but notnecessarily gaseous. In step (2), the flow of the electrolyte, ingaseous and/or liquid form, is stopped. If both liquid and gas flows arepresent, it is preferred that first the gas flow is stopped, and thenthe liquid flow. Step (2) is performed once step (1) is completed.

In step (3), the catalytic electrode is washed with an aqueous liquidwithout applying voltage. Such washing or rinsing is typically performeddirectly before step (4). The washing step is further defined above.

The regeneration may include a step of draining of the cell, or thecatholyte compartment thereof, such that there are no substantialamounts of liquid present. If performed, this is typically done afterstep (3) and before the air is introduced. Even though using a drainedcell for regeneration provides optimal results, in an alternativeembodiment, air may be introduced when the aqueous liquid, as introducedin step (3), is still present. As such, step (4) is performed bybubbling air through the liquid.

During step (4), the actual regeneration by exposure to air takes placeswithout applying voltage. The electrode is contacted with a stream ofair, preferably with a flow rate of 0.01-10 L/min, more preferably0.05-0.5 L/min. The duration of this step may be in the range of 1min-24 h, preferably 0.1-10 h. Optimal results have been obtained withregeneration for about 18 h but also with regeneration for about 1 h intotal (air flow for about 30 min), indicating that the exact duration ofthis step is not crucial. In one embodiment, the duration of step (4) is0.5-1.5 h, most preferably about 1 h.

In step (5), the flow of electrolyte is started again, in order torestart normal operation after the current is reinstated. If both liquidand gas flows are present, it is preferred that first the liquid flow isstarted, and then the gas flow. In step (6), the current is ramped upagain, which is preferably performed with an increase of 0.1-10 mA/min,more preferably 1-4 A/min. Step (6) may be started as soon as theelectrolyte flow(s) has been started. At the end of step (6), theelectrochemical cell is operative again.

The entire regeneration process is preferably performed at or nearambient pressure and temperature, although deviation from theseconditions is possible without significantly affecting the regeneration.In one embodiment, the temperature during the regeneration is in therange of 10-50° C., preferably 15-40° C. Advantageously, the temperatureand pressure during regeneration are the same as those during operationof the electrochemical cell.

Preferably, the regeneration is part of a cyclic process whereinoperation and regeneration are alternated, such as in the processaccording to the invention. The regeneration according to the presentinvention can also be used for activating the catalytic electrode, suchas at the start of an electrochemical conversion.

The Process

The process according to the invention utilizes the regeneration asdefined above.

Herein, step (a) is the operation step of an electrochemical cell,wherein carbon dioxide is converted, and step (b) is the regenerationstep. The process according to the invention can be used to prepareformic acid or formate, such as sodium formate or potassium formate.Whether formic acid or formate is formed, depends primarily on the pHwithin the electrochemical cell. When the pH is below the pKa of formicacid, formic acid will be formed, and when the pH is above the pKa offormic acid, formate will be formed. Typically, the pH will be too highfor the formation of formic acid. The electrochemical conversion of CO₂to formic acid or formate is known to the skilled person. Thisconversion occurs at the cathode. This cathodic reaction can be coupledto any anodic reaction, such as oxygen or chlorine evolution. Theformation of formic acid at the cathode may also be coupled to formationof formic acid at the anode, e.g. by oxidation of glycerol.

Step (a) involves feeding to the cathode an electrolyte comprisingcarbon dioxide. Typically, the carbon dioxide is comprised in acatholyte fed to the cathode, and the process may further comprisefeeding an anolyte to the anode. Usually, the catholyte is fed to afirst cell compartment of an electrochemical cell, comprising thecathode, while the anolyte is fed to a second cell compartment of theelectrochemical cell, comprising the anode. The carbon dioxideconversion to formate/formic acid is typically performed in an aqueousmedium, wherein the CO₂ is bubbled through the aqueous medium ordistributed through the gas-diffusion electrode, e.g. using perculatorsystems.

During step (a), an electrical potential is applied between the anodeand the cathode sufficient to reduce carbon dioxide to formic acid or asalt thereof. The anode is positively charged and the cathodenegatively. In other words, an electrical potential to theelectrochemical cell so that the anode is at a higher potential than thecathode. Cations, typically protons, will thus flow from the anodetowards the cathode where they combine with an oxygen atom liberatedfrom CO₂ to form a water molecule. Electrons, liberated at the anode bythe anodic reaction, are taken up by the anode, while they are generatedat the cathode to be combined with the protons and oxygen atoms intowater molecules and the product of the CO₂ reduction (formic acid orformate). The electrical potential may be a DC voltage. In preferredembodiments, the applied electrical potential is generally between about−1.5 V vs. SCE and about −6 V vs. SCE, preferably from about −1.5 V vs.SCE to about −5 V vs. SCE, such as in the range of −3 V vs. SCE to −5 Vvs SCE and more preferably from about −1.5 V vs. SCE to about −4 V vs.SCE.

It is noted that applying an electrical potential is consideredsynonymous with creating a voltage difference between the cathode andthe anode, so that the anode is at a higher potential than the cathode.The process may be controlled by setting a certain voltage(galvanostatic) or by setting a certain current (potentiostatic). If thevoltage is set, the current will automatically follow from the reactionsthat occur in the cell. If the current is set, the voltage willautomatically follow from the reactions that occur in the cell. Theprocess according to the invention is equally workable in both operationmodes. Typically, the current is controlled in the start-up phase of anelectrochemical cell, in order to find the optimal voltage for thedesired reaction, while during standard operation of the electrochemicalcell, the voltage will be controlled. The process according to theinvention operates with such a voltage difference and/or such a currentthat carbon dioxide is reduced at the cathode.

Preferably, the current density of the electrochemical cell duringoperation is at least 10 mA/cm², such as in the range of 10 mA/cm²-5A/cm², more preferably at least 100 mA/cm², such as in the range 100mA/cm²-3 A/cm². A certain minimal current of at least 10 mA/cm²,preferably at least 100 mA/cm², is preferred in terms of processeconomics, as below these values too little product is formed for aneconomically viable process. The upper limit of the current at which theprocess can operate is solely determined by safety issues. For example,it the current is too high, the cell may heat up too much. Other thanthat, higher currents are preferred since it will result in more productformation. Excellent results have been obtained with a current densityin the range of 100-200 mA/cm². Herein, the currents are defined basedon the projected area of the electrode. The optimal current for theprocess according to the invention may differ based on the exactconditions that are applicable in the electrochemical cell, and theskilled person is able to determine the optimal current in terms ofproduct conversions.

The cathode is the electrode subject to the present invention and asfurther defined above. The anode may be any suitable anode known in theart. The material of the anode is preferably tailored to the desiredanodic reaction, as will be understood by the skilled person.

The reduction of CO₂ to formic acid is known in the art. The halfreaction is typically as follows: CO₂+2H⁺+2 e⁻→HCO₂H [E⁰=−0.20 V vs.RHE]. The carbon dioxide is supplied to the cathode and consumed there.CO₂ can be fed in liquid or gaseous form. The solution of carbon dioxidemay aqueous or non-aqueous and may include buffers such as bicarbonatesand/or phosphates. Non-aqueous electrolytes have been found beneficialin the reduction of CO₂ as the side-reaction at higher potentialswherein H₂ is formed (due to reduction of protons in solution) isreduced. CO₂ gas can also be fed to the cathode compartment through gasdiffusion electrode (GDE). Neutral pH was found to give the best resultsin terms of CO₂ reduction. In one preferred embodiment, the cathode is aGDE and is fed with a gaseous catholyte. In an alternative preferredembodiment, the catholyte is aqueous and liquid catholyte is present inthe cathode compartment. It is well-known to the skilled person toselect specific electrochemical conditions (e.g. the voltage applied andcatholyte composition) in order to optimize the formation of formic acidor formate.

As protons may enter the cathode compartment during operation of theelectrochemical cell, some base may be present to the catholyte. Thus,in one embodiment, the catholyte comprises a base, typically ascontained in a buffer solution, in such an amount to keep the pH of thecatholyte within the cathode compartment neutral. Herein, neutral pHrefers to a pH in the range of 6-8, preferably 6.5-7.5, most preferablyabout 7. The type of buffer solution is not crucial for the operation ofthe electrochemical cell, and a suitable example is potassiumbicarbonate. The skilled person knows how to determine the optimalamount of base, based on the desired pH in the cathode compartment.Since protons are also consumed by the reaction at the cathode, the baseshould not eliminate all protons. By maintaining the pH of the catholytein the desired range, the amount of base will not be excessive.

The CO₂ that is comprised in the catholyte may originate from anysource. In a preferred embodiment, the CO₂ originates from exhaustgases, flue gases or air. Typically, the CO₂ originates from industrialflue gases, such as from power plants or the chemical industry. CO₂ canbe captured from exhaust gases, flue gases and air by methods known inthe art. In case the CO₂ is provided via a gas diffusion electrode asthe electrode according to the present invention, it is preferred thatthe concentration of CO₂ in the gas is as high as possible, such asabove 90 wt %, preferably above 95 wt %, more preferably above 99% wt %or even above 99.9 wt %. In addition to CO₂, some other gaseous speciesmay be present, such as inert gases (N₂, Ar) and/or H₂. The presence ofO₂ in the gas fed to the electrode is preferably avoided.

The process according to the invention may contain a control system thatdetermines when a regeneration step should be performed. This controlsystem determines the performance of the electrochemical cell,preferably by determining the faradaic yield. Step (a) is interruptedand step (b) is initiated in case the performance drops below apredetermined threshold value. As such, regeneration is only initiatedwhen needed, and the performance of the electrochemical cell is furtheroptimized.

The process according to the invention is performed in anelectrochemical cell, preferably the electrochemical cell according tothe invention and further defined below. The process according to theinvention may be a continuous process, preferably wherein a plurality ofelectrochemical cells are connected in parallel and wherein some of thecells are being subjected to the regeneration of step (b) while othercells are simultaneously used for the conversion of step (a). Preferredembodiments for the catalytic electrode as cathode and the regenerationstep are further defined above.

The Electrochemical Cell

The inventors have further designed an electrochemical cell assembly foroperating the process according to the invention. The electrochemicalcell assembly according to the invention is for the continuous reductionof CO₂ to formic acid, comprising a plurality of electrochemical cells,each cell comprising an anode and an indium-containing catalytic gasdiffusion electrode as cathode, wherein the cathode is configured toreceive either an electrolyte containing CO₂ or air, and wherein eachcell further contains an outlet for discharging formic acid or a saltthereof and the assembly is equipped with an air jet stream to enablecontacting of the electrode with air during regeneration.

Each electrochemical cell may contain a cathode compartment and an anodecompartment separated by at least one membrane and wherein the cathodecompartments contains the inlet for receiving either an electrolytecontaining CO₂ or air to the indium-containing catalytic gas diffusionelectrode, and the anode compartment a separate inlet for receiving ananolyte. The membrane may be made from porous glass frit, microporousmaterial, ion exchanging membrane or ion conducting bridge, and allowsionic species to travel from one compartment to the other, such asprotons generated at the anode to the cathode compartment.

The electrochemical cell assembly may contain the plurality ofelectrochemical cells arranged in blocks, wherein each block typicallycontains an equal amount of electrochemical cells, preferably 1-25electrochemical cells, most preferably 1 or 10 electrochemical cells.During operation, each block alternates between a first position whereinit is used for conversion of CO₂ to formic acid or a salt thereof, i.e.step (a) of the process according to the present invention, and a secondposition wherein it is regenerated, i.e. step (b) of the processaccording to the present invention.

EXAMPLES Example 1

CO₂ was converted into potassium formate using an electrochemical cellequipped with a gas diffusion electrode consisting of a high-hydrostatichead gas diffusion layer and a highly active catalytic layer made ofindium/bismuth (50/50 w/w) particles. The catalyst was prepared asdescribed in WO 2019/141827. The gas diffusion electrode was prepared asdescribed in US 2014/0227634 A1. 308 mg of the In-Bi particles weredispersed in 90 mL of isopropanol and stirred for at least 1 hour atroom temperature. 77 mg PVDF (flex binder mass from Kynar) was dissolvedin 90 mL of acetone. The catalyst ink and the PVDF solution were sprayedon the GDL in a layer-by-layer fashion. Care was taken not to flood theGDL during the spraying process. After the spraying, the electrode wasallowed to dry overnight (˜15 h) at room temperature before being usedfor electrochemical testing. The catalyst loading was 1.83 mg/cm² over atotal electrode surface of 168 cm². At the anode, H₂O was oxidized to O₂using an anolyte containing 0.5 M H₂SO₄.

The electrode was tested in three 6-hour runs for a cumulative runtimeof 18 hours, performed at a current density of 150 mA/cm². After eachrun, the electrode was taken out from the cell, rinsed thoroughly withdeionized water and exposed to air overnight at room temperature. Afterthe first run, the colour of the catalytic layer on the electrode turneddarker compared to the as-synthesized electrode (see FIG. 1).

The Faradaic yield during each of the three runs is depicted in FIG. 2.Average values for pH, cell voltage and Faradaic yields per run aregiven in the table below:

Average cell Average Faradaic Run Average pH voltage (V) yield (%) 16.90 +/− 0.34 4.49 +/− 0.10 63.4 +/− 3.9 2 6.51 +/− 0.40 4.33 +/− 0.05 79.5 +/− 13.6 3 6.51 +/− 0.45 4.34 +/− 0.06 87.9 +/− 4.4

The faradaic yield increased from 63% in the first run to 80% in thesecond run and 88% in the third run. Interestingly, the faradaic yieldin the second run started off at around 60% and then steadily increasedto 80%, indicative of activation of the catalyst after the regenerationcycle. Visual inspection of the electrode revealed no salt accumulationin its structure.

Example 2

CO₂ was converted into potassium formate using an electrochemical cellequipped with a gas diffusion electrode consisting of a high-hydrostatichead gas diffusion layer and a highly active catalytic layer made ofindium/bismuth (50/50 w/w) particles. The catalyst was prepared asdescribed in WO 2019/141827. The gas diffusion electrode was prepared asdescribed in US 2014/0227634 A1. 308 mg of the In-Bi particles weredispersed in 90 mL of isopropanol and stirred for at least 1 hour atroom temperature. 77 mg PVDF (flex binder mass from Kynar) was dissolvedin 90 mL of acetone. The catalyst ink and the PVDF solution were sprayedon the GDL in a layer-by-layer fashion. Care was taken not to flood theGDL during the spraying process. After the spraying, the electrode wasallowed to dry overnight at room temperature before being used forelectrochemical testing. The catalyst loading was 0.386 mg/cm² over atotal electrode surface of 233 cm².

The electrode was tested over 7 days with refresh cycles every 20-24hours for a cumulative runtime of 101 hours, performed at a currentdensity of 100 mA/cm² and an active area of 100 cm². At each 20-24 hoursincrement the cell was ramped down at 2 A/min, flow of gas was stopped[depressurized], anolyte and catholyte flows were stopped, water wasintroduced to the front of the cathode for the washing cycle. After 2washes, the air was introduced to the front of the cathode for 20-30minutes. After the each regeneration cycle, the faradic yield wasmaintained. A control run was performed, wherein operation and catalystwas the same as above, except no regeneration cycles were performed. Inthe control run, the faradaic yield gradually declined.

For the process according to the invention, including regenerationcycles, the lapsed time and average pH, cell voltage and Faradaic yieldsper cycle are given in the table below:

Time Average cell Average Faradaic (h) Average pH voltage (V) yield (%)20 6.8 +/− 0.50 4.3 +/− 0.10  96.3 +/− 3 44 6.8 +/− 0.50 4.3 +/− 0.1086.87 +/− 3 67 6.8 +/− 0.50 4.3 +/− 0.10 93.33 +/− 3 74 6.8 +/− 0.50 4.3+/− 0.10 91.91 +/− 3 80 6.8 +/− 0.50 4.124 +/− 0.10  95.11 +/− 3 97 6.79+/− 0.50  4.164 +/− 0.10  95.76 +/− 3 101 6.74 +/− 0.50  4.153 +/− 0.10 95.60 +/− 3

For the control process, without regeneration cycles, the lapsed timeand average pH, cell voltage and Faradaic yields per cycle are given inthe table below:

Time Average cell Average Faradaic (h) Average pH voltage (V) yield (%)17  7.7 +/− 0.50 4.1 +/− 0.10 76.5 +/− 3 24 7.54 +/− 0.50 4.2 +/− 0.1078.1 +/− 3 89 5.22 +/− 0.50 4.1 +/− 0.10 80.4 +/− 3 113  7.4 +/− 0.504.3 +/− 0.10 50.88 +/− 3 

The faradaic yield achieved during the control was consistently below80%, until the end where the yield dropped to 50%. In the regenerationrun, the faradaic yield was consistently above 90%, indicative ofactivation of the catalyst after the regeneration cycle.

Example 3

CO₂ was converted into potassium formate using an electrochemical cellequipped with a gas diffusion electrode consisting of a high-hydrostatichead gas diffusion layer and a highly active catalytic layer made ofindium/bismuth (50/50 w/w) particles. The catalyst was prepared asdescribed in WO 2019/141827. The gas diffusion electrode was prepared asdescribed in US 2014/0227634 A1. 308 mg of the In-Bi particles weredispersed in 90 mL of isopropanol and stirred for at least 1 hour atroom temperature. 38.5 mg PVDF (flex binder mass from Kynar) wasdissolved in 90 mL of acetone. The catalyst ink and the PVDF solutionwere sprayed on the GDL in a layer-by-layer fashion. Care was taken notto flood the GDL during the spraying process. After the spraying, theelectrode was allowed to dry overnight at room temperature before beingused for electrochemical testing. The catalyst loading was 1.62 mg/cm²over a total electrode surface of 253 cm².

The electrode was tested over 7 days with regeneration cycles every20-24 hours for a cumulative runtime of 110 hours, performed at acurrent density of 200 mA/cm² and an active area of 10 cm². At each20-24 hours increment the cell was ramped down at 0.5 A/min, the flow ofgas was stopped [depressurized], the anolyte and catholyte flows werestopped, water was introduced to the front of the cathode for thewashing step. After 2 washes, the air was introduced to the front of thecathode for 20-30 minutes. After the each regeneration cycle, thefaradic yield was maintained at about the same level. A control run wasperformed, wherein operation and catalyst was the same as above, exceptno regeneration cycles were performed (See Example 2). In the controlrun, the faradaic yield gradually declined.

For the process according to the invention, including regenerationcycles, the lapsed time and average values for pH and Faradaic yieldsper cycle are given in the table below:

Time Average Faradaic (h) Average pH yield (%) 17 7.2 +/− 0.50  107 +/−10 41 7.2 +/− 0.50 102 +/− 3 47 7.2 +/− 0.50 100 +/− 3 64 7.2 +/− 0.50100 +/− 3 69 7.2 +/− 0.50 101 +/− 3 90 7.2 +/− 0.50 100 +/− 3 110 7.2+/− 0.50  96 +/− 3

The faradaic yield increased from the control was consistently below80%, until the end where the yield dropped to 50%. In the regenerationrun all faradaic yields were consistently 100%, indicative of activationof the catalyst after the regeneration cycle.

Example 4

CO₂ was converted into potassium formate using an electrochemical cellequipped with a gas diffusion electrode consisting of a high-hydrostatichead gas diffusion layer and a highly active catalytic layer made ofindium/bismuth (50/50 w/w) particles. The catalyst was prepared asdescribed in WO 2019/141827. The gas diffusion electrode was prepared asdescribed in US 2014/0227634 A1. 308 mg of the In-Bi particles supportedon carbon in a ratio of 50% metal: 50% carbon were dispersed in 90 mL ofisopropanol and stirred for at least 1 hour at room temperature. 41.3 mgPVDF (flex binder mass from Kynar) was dissolved in 90 mL of acetone.The catalyst ink and the PVDF solution were sprayed on the GDL in alayer-by-layer fashion. Care was taken not to flood the GDL during thespraying process. After the spraying, the electrode was allowed to dryovernight at room temperature before being used for electrochemicaltesting. The catalyst loading was 0.315 mg/cm² over a total electrodesurface of 250 cm².

The electrodes were tested over 3 days with regeneration cycles every20-24 hours (except control run 1) for a cumulative runtime of 72 hours,performed at a current density of 100 mA/cm² and an active area of 10cm². At each 20-24 hours increment the cell of runs 2-4 were ramped downat 0.5 A/min, the flow of gas was stopped [depressurized], and anolyteand catholyte flows were stopped. In runs 2 and 4, water was introducedto the front of the cathode for the washing cycle. In runs 3 and 4 (forrun 4: after the washing step), air was introduced to the front of thecathode for 20-30 minutes.

The average values for the faradaic yields (%) per cycle, for each ofthe runs, are given in the table below:

Time Run 1 Run 2 Run 3 Run 4 (h) [control] [water only] [air only][water and air] 24 — 56 +/− 5 77 +/− 5 103 +/− 5  48 — 51 +/− 5 77 +/− 588 +/− 5 72 50 +/− 5 42 +/− 5 74 +/− 5 80 +/− 5

The faradaic yield for the control run was at 50% at the end of theexperiment. In the regeneration run with water only and air only, thefaradaic yields were consistently below 80%, a while for theregeneration run with water only and air the faradaic yield wasconsistently above 80%, indicative of activation of the catalyst afterthe regeneration cycle of both water and air being necessary to completethe regeneration.

1. A process for the electrochemical conversion of carbon dioxide toformic acid or a salt thereof, using an indium-containing catalyticelectrode, comprising: (a) electrochemically converting carbon dioxideto formic acid or a salt thereof by applying a voltage to anelectrochemical cell comprising the catalytic electrode as cathode andan anode, wherein the electrochemical cell is fed with an electrolytecomprising carbon dioxide; and (b) regenerating the catalytic electrodeby lowering the voltage and subsequently washing the catalytic electrodewith an aqueous liquid and exposing the catalytic electrode to airwithout applying voltage; and (c) optionally repeating steps (a) and(b).
 2. The process according to claim 1, wherein the catalyticelectrode is an indium-bismuth catalyst, indium-tin catalyst or anindium catalyst.
 3. The process according to claim 1, wherein theprocess is operated in cycles wherein step (c) is repeated at least 10times.
 4. The process according to claim 1, wherein in each cycle theduration of step (b) is 0.1 50% of the duration of step (a); wherein theduration of step (a) in a single cycle is in the range of 1-100 h, andwherein the duration of step (b) in a single cycle is in the range of0.1-2.5 h.
 5. The process according to claim 1, wherein the exposure ofthe electrode to air in step (b) is performed by feeding air to theelectrochemical cell, wherein the electrochemical cell is equipped withair jets.
 6. The process according to claim 1, wherein the catalyticelectrode is a gas diffusion electrode, wherein air is led through thegas diffusion electrode during step (b).
 7. The process according toclaim 1, wherein a plurality of electrochemical cells are connected inparallel and wherein some of the cells are being subjected to theregeneration of step (b) while other cells are simultaneously used forthe conversion of step (a).
 8. The process according to claim 1, whereinthe aqueous liquid is deionized water or the electrolyte used duringstep (a).
 9. The process according to claim 1, wherein a control systemis in place which determines the performance of the electrochemicalcell, by determining the faradaic yield, and wherein step (a) isinterrupted and step (b) is initiated in case the performance dropsbelow a predetermined threshold value.
 10. The process according toclaim 1, wherein feeding of the electrolyte comprising carbon dioxide ofstep (a) involves feeding a liquid and gas flow and step (b) involves:(1) ramping down the current, preferably with a decrease of 0.1-10mA/min; (2) stopping the liquid and gas flows; (3) washing the catalyticelectrode with the aqueous liquid; (4) feeding air at a rate of 0.01 10L/min; (5) starting the liquid and gas flows; (6) ramping up the currentwith an increase of 0.1 10 mA/min.
 11. (canceled)
 12. (canceled)
 13. Anelectrochemical cell assembly for reduction of carbon dioxide to formicacid, comprising a plurality of electrochemical cells, each cellcomprising an anode and an indium-containing catalytic gas diffusionelectrode as cathode, wherein the cathode is configured to receiveeither an electrolyte containing carbon dioxide or washing liquid orair, and wherein each cell further contains an outlet for dischargingformic acid or a salt thereof, wherein the gas diffusion electrodes areequipped with an air jet stream to enable contacting of the electrodewith air during regeneration.
 14. The electrochemical cell assemblyaccording to claim 13, wherein the plurality of electrochemical cellsare arranged in blocks each containing an equal amount ofelectrochemical cells, wherein each block alternates between a firstposition wherein it is used for conversion of carbon dioxide to formicacid or a salt thereof and a second position wherein it is regenerated.15. The electrochemical cell assembly according to claim 13, whereineach electrochemical cell contains a cathode compartment and an anodecompartment separated by at least one membrane and wherein the cathodecompartment contains an inlet for receiving either the electrolytecontaining carbon dioxide or air and the anode compartment a separateinlet for receiving an anolyte.